Xen

In an earlier post I asked about why people like NetBSD and one person
mentioned Xen. Since then I've been browsing the mail archives and I've
seen Xen come up a few times. I'm going to ask a question that might be
tremendously stupid, but if I don't ask I'll never learn:

Isn't Xen just a virtual machine like QEMU, VirtualBox, etc? What makes
Xen special that people here talk about it while they don't talk about
QEMU and VirtualBox?